This invention relates generally to a digital signal processing apparatus, and more particularly to the determination of the background level of a document in a single-pass scanning system.
The features of the present invention may be used in the printing arts and, more particularly, in digital image processing and electrophotographic printing. In digital image processing there is a distinct advantage to being able to determine or detect the background level of a document coincident with the actual scanning of the document. With regard to efficiency, it is advantageous to provide such a feature without having to execute a prescan operation, thereby increasing the throughput of the scanning or digitizing system. Accordingly, the present invention provides a document background detection apparatus that operates during an initial portion of the scanning process to provide a real-time indication of the background level of the document being scanned.
While knowledge of the background level of a scanned document may be useful for numerous image processing operations that could be performed on the video signals generated during scanning, the most apparent use would be for background compensation. Once the background level of the document has been determined, an automated process may be employed to adjust the gain or offset levels of the digitized signals to compensate for a high background level, for example a document produced on a colored sheet of paper.
Another possible use for the background level information is in the determination of the image content in various regions of the document. For instance, a document analysis or image segmentation system might employ the background level information to more reliably detect, from the image signals, the types of images contained on the original document (e.g. text, line art, halftone, and continuous tone). An example of a halftone detection system is found in the patent U.S. Pat. No. 4,811,115, issued to Lin et al. on Mar. 7, 1989 for an "Image Processing Apparatus Using Approximate Auto Correlation Function to Detect the Frequency of Half-tone Image Data", the relevant portions of which are hereby incorporated by reference. Also, a U.S. patent application for "Improved Automatic Image Segmentation", by Shiau et al., Ser. No. 07/722,568 (filed Jun. 27, 1991), hereby incorporated by reference for its teachings, discloses an automatic image segmentation system suitable for classification of various regions of an image having different types of images present therein. Moreover, well known electroreprographic systems like the Xerox.RTM. Docutech Production Publisher,.RTM. may employ image segmentation systems of these types that would benefit from the background level determination capability of the present invention. Other approaches have been devised for the determination of the background level of an image, of which the following disclosures may be relevant:
U.S. Pat. No. 4,970,605, Patentee: Fogaroli et al., Issued: Nov. 13, 1990. PA0 U.S. Pat. No. 4,931,881, Patentee: Matsui et al., Issued: Jun. 5, 1990. PA0 U.S. Pat. No. 4,885,784, Patentee: Miyagawa et al., Issued: Dec. 5, 1989. PA0 U.S. Pat. No. 5,214,471, Patentee: Bares, Issued: May 5, 1993.
The relevant portions of the foregoing references may be briefly summarized as follows:
U.S. Pat. No. 4,970,605 to Fogaroli et al. discloses a digital image scanning device having an automatic background compensation circuit for automatically adjusting a threshold signal in dependence on a plurality of parameters including the background color of an image. An adjusting circuit defines the threshold signal depending on an average of the positive-peak signal and a negative-peak signal previously received from a scanning means.
U.S. Pat. No. 4,931,881 to Matsui et al. discloses an image binarization system having an adjustable binarizing threshold value. The threshold value is based on the average of pixel density values around the pixels to be binarized. The threshold value is corrected downward when the average density of the surrounding pixels is larger than a specific boundary value, and the threshold value is corrected upward when the average density value of the surrounding pixels is smaller than the boundary value.
U.S. Pat. No. 4,885,784 to Miyagawa et al. discloses a method and apparatus for encoding a video image of an object into a binary signal. The method includes analyzing a preliminary video signal in a predetermined window to extract density information, and setting a threshold for binary encoding of subsequent video data using the extracted density information from the preliminary video information.
U.S. Pat. No. 5,214,471 to Bares for a "Background Monitoring Device", the relevant portions of which are hereby incorporated by reference, discloses an apparatus in which background intensity of a copy sheet having visible indicia thereon is monitored. To determine the background level generated by toner powder deposition in non-imaged regions, the sheet is scanned and a signal generated corresponding to the scanned region. The signal corresponding to the scanned region is processed and a signal corresponding to the background intensity of the copy sheet is generated.
The present invention seeks to overcome limitations found in the related references and commercially available products by providing an apparatus for automatically determining the background level of an electronic document, generally consisting of a plurality of video image signals, during the actual scanning or processing of the document. Furthermore, the present invention endeavors to provide the background level determination in a highly accurate manner using a region on the document whose coordinates may be programmed by an external source, thereby increasing the flexibility of the apparatus.
In accordance with one aspect of the present invention, there is provided an image processing system suitable for processing a video image represented by a plurality of video signals. The signals have a predetermined domain of greyscale values and represent both the content and the background regions of the image. The image processing system further includes an apparatus for determining the greyscale level of the background regions. The background determining apparatus includes a windowing circuit for sampling a predetermined portion of the pixel signals in the video image and a comparison circuit for eliminating pixel signals which are not determined to be representative of the background region. Subsequently, an arithmetic logic circuit is employed to calculate a background level based upon the remaining pixel signals.
In accordance with another aspect of the present invention, the background determining apparatus includes a programmable register for input of a default background level which is to be used by the background determining apparatus to output one or more background levels, prior to the completion of the sampling of a predetermined portion of the pixel signals.
In accordance with yet another aspect of the present invention, the background determining apparatus has the capability for computing one or more fractional background levels as a function of the background level, where the additional background levels may be used to generate ranges of video levels for pixel classification.
Pursuant to another aspect of the present invention, there is provided a method for determining the background level of a video image in an image processing apparatus suitable for processing the video image, represented by a plurality of pixel signals. However, only a selected portion of the pixel signals in the video image are sampled for background level determination. As the video image contains pixel signals representing both the content and the background regions of the image, the method next eliminates those sampled pixel signals which are not determined to be representative of the background region. Finally, the remaining pixel signals are used to calculate the background level for the video image.